1. Field of the Invention
The present invention relates to a method of dressing a polishing member, which is used in a polishing apparatus for polishing a workpiece (e.g., an optical parts, a mechanical parts, ceramics, and metal), by a diamond dresser and also relates to a method of determining dressing conditions, a program for determining dressing conditions, and a polishing apparatus. More particularly, the present invention relates to a dressing method, a method of determining dressing conditions, and a program for determining dressing conditions suitable for a polishing pad of a polishing apparatus that polishes a workpiece, such as a semiconductor wafer, to provide a planarized surface, and also relates to such a polishing apparatus.
2. Description of the Related Art
As a more highly integrated structure of a semiconductor device has recently been developed, interconnects of a circuit become finer and dimensions of the integrated device decrease. Thus, it becomes necessary to polish a semiconductor wafer having films (e.g., metal film) or layers on its surface to planarize the surface of the semiconductor wafer. One example of the planarization technique is a polishing procedure performed by a chemical-mechanical polishing (CMP) apparatus. This chemical-mechanical polishing apparatus includes a polishing member (e.g., a polishing cloth or polishing pad) and a holder (e.g., a top ring, polishing head, or chuck) for holding a workpiece, such as a semiconductor wafer to be polished. The polishing apparatus of this type is operable to press a surface (to be polished) of the workpiece against a surface of the polishing member and cause relative movement between the polishing member and the workpiece while supplying a polishing auxiliary (e.g., a polishing liquid, a chemical liquid, slurry, pure water) between the polishing member and the workpiece to thereby polish the surface of the workpiece to a flat finish. It is known that such a polishing process performed by the chemical-mechanical polishing apparatus yields a good polishing result due to a chemical polishing action and a mechanical polishing action.
Foam resin or nonwoven cloth is typically used as a material (raw material) of the polishing member used in such chemical-mechanical polishing apparatus. Fine irregularities (or asperity) are formed on the surface of the polishing member and these fine irregularities function as chip pockets that can effectively prevent clogging and can reduce polishing resistance. However, continuous polishing operations for the workpieces with use of the polishing member can crush the fine irregularities on the surface of the polishing member, thus causing a lowered polishing rate. Thus, a diamond dresser, having a number of diamond particles electrodeposited thereon, is used to dress (condition) the surface of the polishing member to regenerate fine irregularities on the surface of the polishing member.
Examples of the method of dressing the polishing member include a method using a dresser (a large-diameter dresser) that is equal to or larger than a polishing area used in polishing of the workpiece with the polishing member and a method using a dresser (a small-diameter dresser) that is smaller than the polishing area used in polishing of the workpiece with the polishing member. In the method of using the large-diameter dresser, a dressing operation is performed, for example, by pressing a dressing surface, on which the diamond particles are electrodeposited, against the rotating polishing member, while rotating the dresser in a fixed position. In the method of using the small-diameter dresser, a dressing operation is performed, for example, by pressing a dressing surface against the rotating polishing member, while moving the rotating dresser (e.g., reciprocation or swing motion in an arc or a linear vector). In both methods in which the polishing member is rotated during dressing, the polishing area on the surface of the polishing member for use in the actual polishing tends to be an annular area centered on a rotating axis of the polishing member.
During dressing of the polishing member, the surface of the polishing member is scraped off in a slight amount. Therefore, if dressing is not performed appropriately, unwanted undulation is formed on the surface of the polishing member, causing variation (or disorder) in a polishing rate within the polished surface of the workpiece when polishing. Such variation in the polishing rate can be a possible cause of polishing failure. Therefore, it is necessary to perform dressing of the polishing member without generating the undesired undulation on the surface of the polishing member. One approach to avoid the variation in the polishing rate is to perform the dressing operation under appropriate dressing conditions including an appropriate rotational speed of the polishing member, an appropriate rotational speed of the dresser, an appropriate dressing load, and an appropriate moving speed of the dresser (in the case of using the small-diameter dresser).
While the rotational speed of the polishing member, the rotational speed of the dresser, the dressing load, and the moving speed of the dresser can be controlled independently, these elements affect an amount of the polishing member to be scraped off in a complicated manner. In particular, in the dressing operation with use of the small-diameter dresser, determination of the dressing conditions from experiments requires a lot of time and labors. Thus, a method of determining the dressing conditions by simulation has been proposed. For example, Japanese laid-open patent publication No. 10-550 discloses a method of determining a distribution of a sliding distance of a dressing grinder to thereby optimize moving conditions of the dressing grinder. This method utilizes a fact that there is a close relationship between the sliding distance of the dressing grinder at each point on a polishing cloth and an amount of the polishing cloth that has been dressed (i.e., an amount of the polishing cloth scraped off by the dressing grinder).
However, the inventors found out the following. When comparing a simulation result of a distribution of a sliding distance of the diamond dresser and a measurement result of the amount of the polishing pad scraped by the diamond dresser, the simulation is not exactly accurate. FIG. 1 is a view illustrating an example of a movement range of a swinging small-diameter dresser 5 during dressing of a polishing pad 10 which is an example of the polishing member. A dresser arm 17 pivots on a dresser pivot axis O to thereby cause the dresser 5 to swing in a movement range indicated by an arc L. FIG. 2 is a graph showing a measurement result of the amount of the polishing pad scraped off under certain conditions by the small-diameter dresser as shown in FIG. 1 and a distribution of the sliding distances in a radial direction of the polishing pad obtained by a known method. The amount of polishing pad scraped off shown in FIG. 2 is expressed by normalized values which are given by dividing the measurement result of the amount of polishing pad scraped off by an average of the amount of polishing pad scraped off. The sliding distances shown in FIG. 2 are normalized values given by dividing the simulation result of the sliding distance by an average of the sliding distance.
From a quantitative comparison between the amount of the scraped polishing pad and the sliding distance, the followings can be seen. In a region from a center of the polishing pad (where a radius of the polishing pad is zero) to a radius of about 100 mm, both the amount of the scraped polishing pad and the sliding distance increase as the radius of the polishing pad increases. In a region where the radius of the polishing pad is around 120 mm, both the amount of the scraped polishing pad and the sliding distance decrease. In a region where the radius of the polishing pad is larger than 120 mm, both the amount of the scraped polishing pad and the sliding distance increase again. In a region where the radius of the polishing pad is around 250 mm, both the amount of the scraped polishing pad and the sliding distance decrease again. In a region where the radius of the polishing pad is larger than 250 mm, both the amount of the scraped polishing pad and the sliding distance increase again. Thus, there is no doubt that a close relationship exists between the amount of the polishing pad scraped off by the dresser and the sliding distance of the dresser. In this specification, the sliding distance means a travel distance of the dresser at each point on the polishing pad when the dresser and the polishing pad (polishing member) are moved relative to each other while keeping in contact with each other. Specifically, the sliding distance can be given by integrating a relative speed between the each point on the polishing pad and the dressing surface (i.e., the surface with the diamond particles arranged thereon) along a time axis. The aforementioned relative speed is a relative speed when the dressing surface is passing through each point on the polishing pad.
However, in the known method, the simulation result of the sliding distance undulates greatly as shown in FIG. 2, compared with the experimental result of the amount of the polishing pad that has been scraped off. In an accurate simulation of the amount of dressing (i.e., the amount of the polishing pad scraped off by the dressing operation) using the distribution of the sliding distance, the experimental result and the simulation result must be similar in distribution shape thereof. In other words, in FIG. 2, for example, the distribution shape of the amount of the scraped polishing pad and the distribution shape of the sliding distance must be similar to each other (or in a proportional relationship) with respect to the radial direction of the polishing pad. However, as described above, there is a great difference in the distribution shape between them. Therefore, if the known method is used to determine the dressing conditions for a desired amount of the polishing pad to be scraped off with use of the simulation result of the sliding distance, there will be a great difference between the amount of the polishing pad actually scraped off and the desired amount. As a result, further experimental studies are needed to find out dressing conditions that allow a desired distribution of the amount of the scraped polishing pad.
Further, in FIG. 2, the dressing conditions in the experiment and the simulation are such that part of the diamond dresser protrudes from a periphery of the polishing pad. In this case, a contact area between the dresser and the polishing pad decreases since part of the diamond dresser lies out of the polishing pad. As a result, while the dressing load of the diamond dresser (i.e., a load that presses the diamond dresser against the polishing pad) is constant, pressure of the diamond dresser on the polishing pad (i.e., dressing pressure) increases. As the dressing pressure increases, the amount of the scraped polishing pad is expected to increase approximately in proportion to the dressing pressure. In simulation of the sliding distance in FIG. 2, the increase in the dressing pressure is corrected by multiplying the sliding distance by a correction factor. However, as seen in FIG. 2, there is a great difference between the amount of the scraped polishing pad and the simulation result of the sliding distance at the periphery of the polishing pad where the diamond dresser protrudes from the polishing pad.
In a case where the polishing area for use in the polishing operation extends to almost the periphery of the polishing pad, it is necessary to appropriately dress the polishing pad including the periphery thereof. However, as described above, there exists the great difference between the amount of the polishing pad that has been actually removed and the simulation result of the sliding distance at the periphery of the polishing pad. Consequently, further efforts are needed to find out dressing conditions that allow a desired distribution of the amount of the scraped polishing pad for that purpose.
In addition, as the semiconductor device becomes smaller and the interconnects become finer, an acceptable range of the variation in the polishing rate decreases and it becomes important to appropriately control the distribution of the amount of the scraped polishing pad that affects the variation in the polishing rate. Therefore, it is necessary to determine the dressing conditions using a more accurate simulation.